<!DOCTYPE html>
<html>
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="user-scalable=no,width=device-width,initial-scale=1.0, maximum-scale=1.0">
  <title>栅格系统布局</title>
  <style>
  .row {
    width: 100%;
  }
  /* 通过伪元素:after清除浮动 */
  .row :after {
    clear: left;
    /* 清除左浮动 */
    content: '';	
    display: table;
    /* 该元素会作为块级表格来显示（类似 <table>）*/
  }
  /* CSS3新增[attribute^=value]选择器，用于匹配属性值以指定值开头的每个元素 */
  [class^="col"] {
    float: left;
    background-color: #e0e0e0;
  }
  .col1 {
    width: 25%;
  }
  .col2 {
    width: 50%;
  }
  @media (max-width: 768px) {
    .row {
      width: 100%;
    }
    [class^="col"] {
      float: none;
      width: 100%;
    }
  }
  </style>
</head>
<body>
  <div class="row">
    <header>页头</header>
  </div>
  <div class="row">
    <nav class="col1">导航</nav>
    <div class="col2">主要内容</div>
    <aside class="col1">侧边栏</aside>
  </div>
  <div class="row">
    <footer>页尾</footer>
  </div>
</body>
</html>
